This invention relates to an image projector which enlarges and projects an image appearing on a CRT (cathode ray tube), etc., which is defined herein as an image source, through projection lenses onto a rear projection type screen.
This type of image projector is disclosed, for example, in Japanese Patent Provisional Publication Sho62-31838, in which a video projector is illustrated in FIG. 1. As shown, this prior art projector has a CRT 2, on which an optical image appears, a projector lenses 3, and a reflecting mirror 4 within a casing 1. On one side of the casing 1, a rear projection type screen 5 is arranged.
In the above described projector, the reflecting mirror 4 is formed to have a function of a convex mirror, with a diverging effect that permits the screen 5 to be brought in the proximity of the reflecting mirror 4, thereby making the overall thickness of the projector smaller. However, in the above described projector, an angle .theta. between the reflecting mirror 4 and the screen 5 must be 45 degrees, which causes a limit to making the size thin.
In order to further reduce the size, the above angle .theta. has to be made smaller. In this connection, theoretically, a projector as illustrated in FIG. 2, can be constructed wherein a chart surface 10 of the image source 2 and a projector lenses 20 are arranged to be inclined with respect to a reflecting surface 30.
Assume an orthogonal coordinate with the point of intersection 0 between an optical axis l1 of the projection lenses 20 and the reflecting surface 30 as being the origin. Let the axis passing through the point of intersection 0 and crossing the reflecting surface 30 at right angles be an x-axis; then let the crossed line between the surface including optical axis l1 and the x-axis and the reflecting surface 30 be a y-axis, and the axis crossing both x and y axes at right angles be a z-axis. Namely, the reflecting surface 30 coincides with y-x plane and FIG. 2 shows the cross-section along the x-y plane. In this reference example, the chart surface 10 is set up perpendicular with respect to optical axis l1.
Hereunder, an example is described with reference to actual numerical values. The optical system given in the example below is intended to magnify an optical image by 12 times the original formed on the chart surface 10 of the image source 2 and project it on the screen 40 using the optical image with a height of +/-22.860 mm in the y direction and a height of +/-30.480 mm in the z direction on the chart surface 10.
All the angles in the following description concern those on this x-y plane; the clockwise direction with respect to the reference direction is set to be "-".
The projector lenses 20 employed in the projector illustrated in FIG. 2 may comprise, for example, a 4-group/6-leaf lens system as illustrated in FIG. 3, the configulational relation of which is indicated in Table 1.
TABLE 1 ______________________________________ R.sub.1 = 66.838 D.sub.1 = 7.340 N.sub.1 = 1.72342 UH.sub.1 = 25.00 R.sub.2 = -351.917 D.sub.2 = 0.200 N.sub.2 = 1.00000 UH.sub.2 = 25.00 R.sub.3 = 33.209 D.sub.3 = 9.700 N.sub.3 = 1.72000 UH.sub.3 = 20.00 R.sub.4 = -118.902 D.sub.4 = 2.500 N.sub.4 = 1.80518 UH.sub.4 = 20.00 R.sub.5 = 26.650 D.sub.5 = 13.270 N.sub.5 = 1.00000 UH.sub.5 = 15.00 R.sub.6 = -27.832 D.sub.6 = 2.500 N.sub.6 = 1.62004 UH.sub.6 = 15.00 R.sub.7 = 189.738 D.sub.7 = 9.240 N.sub.7 = 1.72000 UH.sub.7 = 20.00 R.sub.8 = -39.088 D.sub.8 = 0.200 N.sub.8 = 1.00000 UH.sub.8 = 25.00 R.sub.9 = -212.500 D.sub.9 = 5.450 N.sub.9 = 1.72342 UH.sub.9 = 25.00 R.sub.10 = -71.903 UH.sub.10 = 25.00 ______________________________________
In Table 1, as indicated in FIG. 3, R1, R2, . . . R9 and R10 are the radius of curvature surfaces from the side of the chart surface 10, and UH1, UH2, . . . UH9 and UH10 are the apertures thereof, D1, D2, . . . D8 and D9, are the distances between respective curvature surfaces, and N1, N2 . . . N8 and N9 are the refractive indexes of the medium within the range corresponding to the respective distances.
In this specification, the projection lenses with a focal distance of 75 mm formed as above is defined as an actual lens in order to distinguish it from an ideal lens to be described later.
In FIG. 2, .theta.1 is the tilt angle of the reflecting surface 30 with respect to the screen 40, and .theta.2 is the tilt angle of the optical axis l1 with respect to the x-axis. The distance from the chart surface 10 along the optical axis l1 to the first curvature surface of the projection lens 20 is d1. The distance from the final curvature surface of the lens 20 to the origin 0 is d1. The distance from the origin 0 along the reflecting light pass l2 of the optical axis l1 of the reflecting surface 30 to the screen 40 is d3.
The specifications in case that an actual lens is used are as per the values given in Table 2. The light path in this case is as shown in FIG. 4A and the distortion and the spot diagram of the image projected on the screen 40 are as shown in FIG. 4B.
TABLE 2 ______________________________________ .theta..sub.1 = 37.000* d.sub.1 = 66.506 mm .theta..sub.2 = 37.000* d.sub.2 = 661.550 mm d.sub.3 = 225.000 mm ______________________________________
FIG. 4A shows five main beams with heights in the y direction on the chart surface 10 of -22.860 mm, -11.430 mm, 0.000 mm, 11.430 mm and 22.860 mm, respectively, with respect to the origin 0, and two adjacent beams each for respective main beams.
FIG. 4B shows a case where a 6.times.8 mesh is set up so that the vertical and horizontal pitch is 7.62 mm on the chart surface 10 and this mesh is projected on the screen 40 via the above-mentioned optical system: it is desirable to bring it to the 6.times.8 reference mesh (shown by a broken line in the figure) as close as possible so that the chart surface 10 is enlarged by 12 times or so that the vertical and horizontal pitch becomes 91.44 mm. In the optical system shown in this example, as the value of .theta. is large, it is possible to almost completely coincide the mesh to be actually projected with the reference mesh.
The spot diagram is shown as a collection of points in the figure. In this figure, simulation is done using 100 beams per spot diagram projected from the same location on the chart surface 10 and the spot scale is indicated as 10 times the scale of the mesh. As the spot state on the right half region is symmetrical with that on the left half, illustration on the left half is omitted.
Next, the thickness of the projector is explained. The thickness is given as the distance between the screen 40 and the lower end of the reflecting surface 30 as a reference. Now, let the lower end of the reflecting surface 30 be point of an intersection P (refer to FIG. 4A) between the main beam emitted from y =-22.880 mm on the chart surface 10 and the reflecting surface 30, if an imaginary line passing through this P and crossing with the screen 40 at right angles is set up, the screen 40 and the distance between the point of intersection of this imaginary line and P are used as a reference with which to indicate the thickness of the projector. In this specification, this distance is defined as a thickness index d.sub.T.
In the above example, d.sub.T =356.531 mm.
In the example above, an actual lens is used as the projection lenses 20. For reference, design values and performance, when a so-called ideal lens, which does not cause aberration is employed are given below.
Table 3 indicates each tilt angle, and distance. The definition of codes are the same as those in the above example. The thickness of an ideal lens is assumed to be 0 (Zero).
The light path is as given in FIG. 5A and the distortion and the spot diagram of the projected image are given in FIG. 5B.
TABLE 3 ______________________________________ .theta..sub.1 = 37.500* d.sub.1 = 81.250 mm .theta..sub.2 = 37.500* d.sub.2 = 755.000 mm d.sub.3 = 220.000 mm ______________________________________
The mesh as projected on the screen 40 completely coincides with the reference mesh. FIG. 5B is a simulational diagram of spots corresponding to 100 beams emitted from a single point on the chart surface 10 in the same manner as that in FIG. 4A. As an ideal lens is employed, the spots converge completely on a single point on the screen 40 and are not indicated as diverging spots in the figure unlike that in FIG. 4B.
The thickness index in this configuration will be d.sub.T =356.531 mm the same as in the above example.
In the example mentioned above, however, if the tilt angle .theta.1 is less than 37 degrees, image performance will degrade.
A modified example is explained hereunder with reference to FIG. 8. Although FIG. 8 is a configurational diagram to explain the first embodiment of this invention, as the basic configuration is the same as that for the modified example, this figure is used to explain the configuration in the following example. The same symbols are used for the parts identical to those in FIG. 2 and the definitions of .theta.1, .theta.2, d1, d2 and d3 shown in the diagram are the same as those in the example mentioned above.
In the example below, the chart surface 10 is inclined with respect to the surface crossing the optical axis l1 at right angles, the tilt angle of which is defined as .theta.3. In this way, by tilting the chart surface 10 counterclockwise with respect to the above-mentioned surface crossing at right angles, the distance between the projection lenses 20 and the imaging point of beams reflected by the upper part of the reflecting mirror 30 can be made longer than the distance between the projection lenses 20 and the imaging point of beams reflected by the center of the reflecting surface 30. At the same time, the distance between the projection lenses 20 and the imaging point of beams reflected by the lower part of the reflecting surface 30 can be made shorter than the distance between the projection lenses 20 and the imaging point of beams reflected by the center of the reflecting surface 30, whereby the variance of the imaging point corresponding to each point on the chart surface 10 from the screen 40 can be corrected.
In the configuration in FIG. 8, each tilt angle, and distance when an actual lens (lens shown in Table 1) is used are as per Table 4. In this case, the optical path is as shown in FIG. 6A and the distortion and the spot diagram is shown in FIG. 6B.
Thickness index d.sub.T =282.617 mm.
TABLE 4 ______________________________________ .theta..sub.1 = 16.499* d.sub.1 = 66.506 mm .theta..sub.2 = 56.500* d.sub.2 = 606.550 mm .theta..sub.3 = 4.000* d.sub.3 = 280.000 mm ______________________________________
Each tilt angle and distance when the above-mentioned ideal lens is used as the projection lens 20 for the configuration in FIG. 8 are as shown in Table 5. In this case, the optical path is as shown in FIG. 7A and the distortion and the spot diagram are as shown in FIG. 7B. The spot diagram can be made complete just as in FIG. 5B.
Thickness index d.sub.T =286.965 mm.
TABLE 5 ______________________________________ .theta..sub.1 = 17.499* d.sub.1 = 81.250 mm .theta..sub.2 = 57.500* d.sub.2 = 695.000 mm .theta..sub.3 = 4.000* d.sub.3 = 280.000 mm ______________________________________
Although the mechanical configuration can be made thinner if the angle between the screen 40 and the reflecting surface 30 is made smaller, the image distortion as projected on the screen 40 will be large as shown in FIG. 7B with the result that the system cannot withstand practical use.
As is apparent from the above explanations, in the case where the optical axis of the projection lenses is inclined with respect to the screen, the image projection performance is considerably reduced due to the distortion of the image and/or defocusing of the image caused by the inclination of the optical axis of the projection lenses.